Field of the Invention
The invention concerns simultaneous multislice magnetic resonance techniques.
Description of the Prior Art
Magnetic resonance (MR) is a known modality that can be used to generate images of the inside of an examination object. Simply described, the examination object is moved into a magnetic resonance scanner in which the object is subjected to s strong static, homogeneous constant basic magnetic field, also called a B0 field, with field strengths of 0.2 Tesla to 7 Tesla and more, such that nuclear spins in the object are oriented along the constant magnetic field. To trigger nuclear spin resonances, radio-frequency (RF) excitation pulses are radiated into the examination object, the triggered MR signals are measured as so-called k-space data, and on the basis thereof MR images are reconstructed or spectroscopy data are determined. For spatially encoding the measurement data, rapidly switched magnetic gradient fields are overlaid on the constant magnetic field. The recorded measurement data are digitized and stored in the form of complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the populated k-space matrix, for example by a multidimensional Fourier transform.
The most frequently used method for generating echo signals after exciting the nuclear spins is what is known as the spin echo (SE) method. In the simplest case, by radiating at least one RF refocusing pulse after the RF excitation pulse has been radiated, the transverse magnetization of the nuclear spins is “turned,” so dephased magnetization is rephased again and thus a signal known as a spin echo SE, is generated after a time TE referred to as echo time that follows on from the RF excitation pulse.
The excitation and the measurement of the generated echo signals are repeated after a repetition time TR (e.g. by switching various gradients for spatial encoding) until a number of echo signals has been measured and stored in k-space, in order to be able to map the desired region examination object that is to be imaged.
Among the SE sequences, the TSE sequences (TSE: “Turbo Spin Echo”), which are also known by the names FSE (“Fast Spin Echo”) and RARE (“Rapid Acquisition with Refocused Echoes”) sequences, are common in clinical applications. The advantage of the TSE sequences compared with the “simple” SE sequence is that a number of refocusing pulses are radiated after an RF excitation pulse. As a result, a number of spin echo signals also generated after an excitation. As a result, the data acquisition is accelerated because fewer repetitions of the sequence with different position encoding are required in order to measure all desired data. With TSE sequences, the measurement time for the entire k-space is thus reduced compared with conventional SE methods, according to the number of echo signals that are refocused and acquired after an excitation; this is known as the “turbo factor”.
However since 180° pulses are conventionally used for the refocusing pulses, TSE sequences can generate a high SAR load (“specific absorption rate”) load. It is also possible to use smaller flip angles for the refocusing pulses without significantly reducing the signal intensity, so a “pseudo steady state” is generated. This is described e.g. in the article by Alsop “The Sensitivity of Low Angle RARE Imaging”, Magnetic Resonance in Medicine, 37, 1997, pages 176-184. How the flip angles can be selectively manipulated is described by Hennig et al. in “Multiecho Sequences with Variable Refocusing Flip Angles: Optimization of Signal Behavior Using Smooth Transitions between Pseudo Steady States (TRAPS)”; Proc. Intl. Soc. Mag. Reson. Med. 10, 2002, page 2356.
A further option for reducing the time required for a complete measurement is the use of driven equilibrium methods. With driven equilibrium methods, after an echo signal has been acquired, in particular the last echo signal within a repetition, at least one further RF pulse is radiated into the examination object, which serves to rapidly reestablish the longitudinal magnetization prior to the next excitation (T1 relaxation), and thus permits shorter repetition times TR without negatively affecting the contrast.
Driven equilibrium methods are described for instance in the article by van Uijen et al., “Driven-Equilibrium Radiofrequency Pulses in NMR Imaging”, Magnetic Resonance in Medicine 1, 1984, pages 502-507 and in U.S. Pat. No. 4,893,081.
The desire for ever-quicker MR recordings in a clinical environment is currently leading to widespread development and use of methods in which a number of images are recorded simultaneously. These methods can generally be characterized by the way in which transverse magnetization of at least two slices is selectively used at the same time for the imaging process (“multislice imaging”, “slice multiplexing”) at least during part of the measurement. In contrast, with established “multislice imaging”, the signal of at least two slices is recorded alternately i.e. completely independently of one another with a correspondingly longer measurement time.
Known methods here are, for instance, Hadamard encoding, methods with simultaneous echo refocusing, methods with broadband data acquisition, and methods that use parallel imaging in the slice direction. The latter methods include the CAIPIRINHA technique, such as described by Breuer et al. in “Controlled Aliasing in Parallel Imaging Results in Higher Acceleration (CAIPIRINHA) for Multi-Slice Imaging”, Magnetic Resonance in Medicine 53, 2005, pages 684-691, and the blipped CAIPIRINHA technique, as described by Setsompop et al. in “Blipped-Controlled Aliasing in Parallel Imaging for Simultaneous Multislice Echo Planar Imaging With Reduced g-Factor Penalty”, Magnetic Resonance in Medicine 67, 2012, pages 1210-1224.
With slice multiplexing methods of this type, a type of excitation pulse known as a multiband RF pulse is used, in order to excite nuclear spins in two or more slices at the same time, or otherwise to manipulate them, e.g. to refocus or saturate them. Such a multiband RF pulse can be a multiplexed output of individual RF pulses, which are used to act on the individual slices to be manipulated at the same time. In order to be able to separate the resulting signals of the various slices, a different phase is applied in each case to the individual RF pulses prior to the multiplexing for instance, such as by adding a linear phase increase, as a result of which the slices in the spatial domain are displaced with respect to one another. By multiplexing, a baseband-modulated multiband RF pulse is obtained by adding the pulse shapes of the individual RF pulses, for instance.
As described in the article by Setsompop et al. cited above, g-factor disadvantages can be reduced by displacements between the slices, such as by gradient blips being used or the phases of the individual RF pulses being modulated accordingly. As likewise described in the cited article by Setsompop et al. as well as in the cited article by Breuer et al., the signals of the slices that are excited at the same time, or are otherwise manipulated, can be combined as signals from just one slice, in order then to be separated in post-processing by a parallel reconstruction method, e.g. a (slice) GRAPPA Method (GRAPPA: “GeneRalized Autocalibrating Partial Parallel Acquisition”) or a SENSE method (SENSE: “SENSitivity Encoding”).